Curable norbornenyl functional silicone formulations

ABSTRACT

A composition cureable to a solid crosslinked polyorganosiloxane comprises: 
     a) a norbornene functional silicone prepolymer having a plurality of groups of the formula: ##STR1##  bound to silicone atoms thereof, where R is H or methyl, R 1  is a direct bond of --C(═O)O-- and R 4  is ethylene, ethenylene, propylene or propenylene, 
     b) a coreactive silicone prepolymer selected from SiH functional silicones, and alkylthiol functional silicones, said polyorganosiloxanes (a) and (b) being present in an equivalent ratio of between 2:1 and 1:2 norbornene to silicon hydride or alkyl thiol groups, and 
     c) an effective amount of a hydrosilation or thiol-ene cure catalyst, respectively.

This application is a continuation-in-part of application Ser. No.099,676, filed Sept. 21, 1987, now U.S. Pat. No. 4,808,638, and acontinuation-in-part of Ser. No. 917,962, filed Oct. 14, 1986, nowabandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

In co-pending application Ser. No. 099,676, there are described curablethiol-ene compositions using norbornenyl functional olefins. Theinvention of the present application pertains to silicone formulationsutilizing particular norbornene functional silicone prepolymers whichare cured by a thiol-ene or hydrosilation reaction mechanism.

2. Description of the Prior Art

In Petrov, et al, Zhurnal Obschei Khimii, 31, 1199 (1961), there is adescription of the hydrosilation of various norbornene compounds, one ofwhich is vinyl norbornene. The vinyl norbornene reaction was run a 30°C. using trichlorosilane. The authors of this reference report that amixture of products was obtained, the addition taking place at the boththe internal double bond of the norbornene ring and at vinyl group inapproximately similar proportions: ##STR2##

In various references, Watanabe et al, describe hydrosilation reactionsof 5-ethylidenebicyclo (2,2,1)hept-2-ene: ##STR3## This hydrosilation isonly reported to produce addition on the norbornene ring double bond.See for instance U.S. Pat. Nos. 4,599,440; 4,623,741; 4,625,043; and4,640,968. These reaction products were used to make siloxane monomersfor vulcanization to yield silicone elastomers.

From the Petrov et al and Watanabe et al, references persons skilled inthe art would conclude that it is not possible to selectivelyhydrosilate norbornene derivatives containing other unsaturated sitesbecause the norbornene unsaturation itself is reactive underhydrosilation conditions.

SUMMARY OF THE INVENTION

The present invention pertains to novel cureable polyorganosiloxane("silicone") formulations utilizing norbornene functional silicones asthe polyene in a thiol-ene or hydrosilation cured formulation.

In particular the cureable formulations of the invention comprise:

a) a norbornene functional silicone prepolymer having a plurality ofgroups of the formula: ##STR4## bound to silicon atoms thereof, where Ris H or methyl, R¹ is a direct bond or --C(═O)O-- and R⁴ is ethylene,ethenylene, propylene or propenylene,

b) a coreactive silicone prepolymer selected from SiH functionalsilicones, and alkylthiol functional silicones, said polyorganosiloxanes(a) and (b) being present in an equivalent ratio of between 2:1 and 1:2norbornene to silicon hydride or alkyl thiol groups, and

c) an effective amount of a hydrosilation or thiol-ene cure catalyst,respectively.

Particularly preferred formulations are free radically cured thiol-enesystems, i.e. where (b) is an alkylthiol functional silicone and (c) isa free radical initiator such as a peroxide or a radical photoinitiator.

DETAILED DESCRIPTION OF THE INVENTION

The norbornene functional silicones used in the formulations of thepresent invention are prepared by a novel process comprising the stepsof:

1) reacting a norbornene compound of the formula: ##STR5## where R is Hor methyl, R¹ is a direct bond or --C(═O)O-- and R² is vinyl, ethynyl,allyl or propargyl, with a silicon hydride functional compound in thepresence of hydrosilation catalyst at a temperature of at least 60° C.;and

2) recovering a product which consists essentially of the additionproducts of the silicon hydride functional compound to the R² group ofthe norbornene compound and which is substantially free of additionproducts of the silicon hydride functional compound to the internal ringdouble bond of the norbornene compound.

The reaction to selectively hydrosilate the side chain unsaturation ofthe starting norbornene compound is surprisingly straightforward. Usingreaction temperatures of at least 60° C., preferably at least 70° C., inthe presence of common hydrosilation catalysts, the reaction proceedsexothermically. It is generally desirable that the norbornene compoundbe in substantial excess on the basis of moles of norbornene compared toequivalents of SiH. Suitably there is a 50% excess. The excessnorbornene compound, however, appears desirable more from the standpointof improving yield by driving the reaction toward completion rather thanas a safeguard against non-selective hydrosilation of the norbornenering double bond. Upon completion of the reaction, the excess norbornenestarting compound is readily removed from the reaction mixture bydistillation.

The selective hydrosilation reaction to produce the norbornenefunctional silicones will be described and exemplified below primarilywith respect to the preferred vinyl norbornene starting material.However, it will be readily seen that similar results may be obtainedusing other norbornene compounds within the formula given above.Moreover it will be appreciated that modifications of the materials andconditions exemplified herein may readily be made by those skilled inthe art.

SiH functional organosiloxane polymers can be used to directlyhydrosilate the norbornene compound. Suitable procedures may be obtainedby modifying the examples of U.S. Pat. No. 4,665,147, substitutingequivalent weights of vinyl norbornene for the β-(allyloxy)ethylmethacrylate used in these examples. However, it will generally be moreconvenient to use SiH functional silanes which also contain hydrolyzablefunctionality to prepare norbornene functional silanes which alsoinclude one, two or three hydrolyzable groups bound to the silicon atomthereof: ##STR6## Such silane compounds serve as useful monomers orcapping agents for organosiloxane polymers whose molecular sizes, extentof branching and distribution of functional groups may be designed toprovide specific desirable properties in the resulting norbornenefunctional prepolymer or in a cured polymer thereof. Examples ofsuitable hydrolyzable groups include chloro, methoxy, ethoxy, oxime suchas methyl ethyl ketoximino, acetoxy, N,N-dialkylamino, and otherhydrolyzable groups described in U.S. Pat. No. 4,699,802. For mostorganosiloxane polymerization or capping reactions methoxy or chlorogroups will be satisfactory.

Hydrosilation catalysts are well known to those skilled in the art.Examples are platinum, chloroplatinic acid, hydrocarbon platinumcomplexes, rhodium complexes, etc. Platinum based catalysts, such asKarstedt catalyst and chloroplatinic acid, are preferred at levels ofbetween 10 ppm and 500 ppm platinum, more preferably between 50 ppm and300 ppm. The reactions can be carried out neat or in solvents which donot interfere with hydrosilations. Toluene, hexane, tetrahydrofuran,methylene chloride and benzene are examples of suitable organicsolvents. The hydrosilation reactions can be followed by observing thedisappearance of the SiH absorption peak at 2200 cm⁻¹ of the infraredspectrum. Normally the reactions are complete within three hours.

The thiol-ene compositions of the invention preferably comprise:

a 2-(2-norborn-5-enyl)ethyl terminated polydimethylsiloxane polymer ofthe formula: ##STR7## where q is from 100-1500;

an equivalent weight amount of organosiloxane compounds havingalkylthiol functionality, at least 20 percent (equivalent basis) of thealkylthiol functional organosiloxane being a polythiol compound of theformula: ##STR8## where R⁷ is lower alkylene, suitably ethylene, x is3-10 and y is 25-50; and,

an effective amount of an initiator of thiol-ene addition reactions,suitably a free radical photoinitiator.

Still more preferably the alkylthiol functional component is a mixturewhich consists of essentially of a polythiol compound (VI) and anoligomeric dithiol chain extender of the formula: ##STR9## where m isbetween 1 and 3 and n is 3-15.

When cured elastomers having high elongation and low durometer valuesare desired, inclusion of the bis(mercaptoalkyl)dimer in lieu of part ofthe polymercaptoalkylsiloxane crosslinker allows such properties to beobtained from much lower viscosity formulations. This reduces the needfor high molecular weight, high viscosity components which exacerbateformulation and application difficulties.

The most preferable values are as follows:

V: q is 380

VI: x is 5 and y is 30

VII: n is 3 or 4 and m is 1

Photoinitiator: 1.5 wt % diethoxyacetophenone.

Best results are obtained when the composition conforms to the thefollowing constraint: ##EQU1##

Suitably component VII is1,3-bis(3-mercaptopropyl)-1,1,3,3-tetramethyldisiloxane, which may beprepared by a modification of a typical 3-mercaptopropyltrimethoxysilanesynthesis. 1,3-Bis(3-chloropropyl)-1,1,3,3-tetramethyldisiloxane may bereacted with thiourea and ammonia to give the aforementioned product.

Capping reactions to produce ethyl norbornene functional silicones suchas the silicones of formula V are suitably accomplished by condensing anethylnorbornene silane having hydrolyzable functionality as in formulaIV with a silanol terminated diorganosiloxane polymer of the formula:##STR10## where p is between 100 and about 1500 and R⁵ and R⁶ are thesame or different organo groups. Suitable R⁵ and R⁶ groups arehydrocarbyl or substituted hydrocarbyl groups including alkyl such asmethyl, ethyl, cyclohexyl, etc.; haloalkyl such as3,3,3-trifluorolpropyl; and aryl which may be optionally substitutedwith hydrocarbon or halogen groups, especially phenyl, benzyl ormethylphenyl. More preferably R⁵ and R⁶ are methyl or phenyl, mostpreferably methyl. The silanol terminated silicones typically haveviscosities in the range of 500 centipoise (mPas) to 120,000 centipoise(mPas), preferably between 750 and about 50,000 centipoise (mPas), stillmore preferably between about 2,000 and 30,000 centipoise (mPas). Thecapping reaction is run in the presence of a conventional condensationcatalyst, suitably a titanate, tin or tertiary amine catalyst.

Silanes of formula IV may also be used as monomers to prepare norbornenefunctional homopolymers and copolymers by conventional condensationpolymerization procedures. Furthermore, as described in example 4 below,silanes of formula IV may also be used to prepare norbornene functionalcyclic oligomeric siloxanes which likewise can be used norbornenefunctional siloxane polymers and copolymers by conventional methods.

The initiator used in the cureable thiol-ene formulations is suitably afree radical photoinitiator. Examples of free radical photoinitiatorsinclude benzoin and substituted benzoin compounds, benzophenone,Michler's ketone, dialkoxybenzophenones, dialkoxyacetophenones,peroxyesters described in U.S. Pat. Nos. 4,616,826 and 4,604,295, etc.Photosensitizers made compatible with silicones by bindingphotoinitiating groups to organosiloxane polymer backbones, such as thecompounds disclosed in U.S. Pat. Nos. 4,477,326, 4,507,187, 4,587,276,4,534,838 and 4,666,953 may also be used. Alternatively, thermal freeradical initiators such as peroxy or azonitrile initiators can be usedto cure the formulations.

Salts of complex halogenides known as cationic photoinitiators may alsobe used to cure the thiol-ene formulations. Examples of such cationicphotoinitiators include di-ρ-tolyliodonium hexafluorophosphate,diphenyliodonium hexafluoroarsenate and UVE 1014 (trademark of GeneralElectric Co.), a commercially available sulfonium salt of a complexhalogenide.

Combinations of organic peroxides and certain η⁵ η⁶ -iron arene complexcompounds as described in U.S. Pat. No. 4,808,638, incorporated hereinby reference, may also be employed as photoinitiators.

Typically the cureable formulations of the invention will also includeone or more fillers. Suitable fillers are reinforcing glass fiber orsilica fillers. Particularly preferred are fumed silicas, especiallyfumed silicas which have been treated to render them hydrophobic. Suchsilicas can be added at high levels, sometimes 60% or more, whilemaintaining practical viscosities. Especially preferred are silicastreated to give low thixotropic ratios such as Wacker-Chemie HDK-2000™.For most applications such fillers will desireably be employed at levelsbetween about 1% and 60%, suitably between about 10% and 40%, preferably25%-35%.

Inert or semi-reinforcing fillers may also be employed such as groundquartz, calcium carbonate, talc, clay and their treated counterparts, aswell as other materials well known in the art. Levels of such fillers,when used, are typically 5%-60% based on the total weight of theformulation.

Although silicon hydride functional compounds will selectivelyhydrosilate the side unsaturated groups of the norbornene startingcompounds under the conditions described herein, the ring unsaturationis reactive to hydrosilations. Thus, the norbornene functionalpolyorganosiloxane component of the inventive compositions mayalternatively be cured using a conventional hydrosilation cure system.In such systems a second silicon hydride functional compound is used,after synthesis of the norbornene silicone, to cure the norbornenesilicone by SiH addition to the norbornene ring. Hydrosilation catalystsand their concentrations are as previously described. Thepolyorganosiloxane backbone of the SiH functional curing silicone may bethe same as, or different from, the polyorganosiloxane backbone of thenorbornene functional silicone. Preferably the SiH curing silicone isone which includes more than two SiH groups per molecule. Example 8illustrates this type of hydrosilation cureable formulation.

Suitably the SiH functional silicones used in hydrosilation curedformulations are similar to the silicones of formula VI, above, exceptthat the R⁷ --SH groups are replaced by H. Likewise, difunctional SiHcompounds analogous to the dithiols of formula VII may also be employedto extend the length of the cured polymer between crosslinks.

While a cureable composition using norbornene functional silicones ofthe invention may include both difunctional norbornenyl silicones andeither a difunctional SiH or a difunctional alkylthiol silicone, it willbe understood that at least a portion of at least one of thesecomponents should contain more than two functional groups per moleculeto produce a crosslinked product when cured. That is, the total of theaverage number of norbornene groups per molecule of norbornenefunctional prepolymer and the average number of coreactive groups (thiolor SiH) per molecule of the coreactive prepolymer should be greater than4 when a crosslinked cured product is desired. This total is referred toas the "total reactive functionality" of the composition.

The invention is illustrated by the following non-limiting examples.

EXAMPLE 1 2-(Norborn-5-enyl)ethyltrichlorosilane

5-Vinyl-2-norbornene (1.23 M, 148 g) was stirred under nitrogen at 75°C. in a round-bottomed flask equipped with a condenser and a pressureequalizing addition funnel. Trichlorosilane (0.616 M, 83.11 g) was addeddropwise and Karstedt catalyst (0.23 g) was quickly added. The rate ofaddition was adjusted to maintain a 75°-80° C. pot temperature withoutexternal heating. When addition was complete, the reaction mixture wasaged at 75° C. for three hours. Infrared analysis indicated no silanewas present. The reaction mixture was then concentrated on a rotaryevaporator and distilled in vacuo (bp 55°-62° C., 0.05 mm Hg). Yield143.4 g (91% theory, >95% purity by GLPC).

Similar results are obtained when chloroplatinic acid is used as thecatalyst.

EXAMPLE 2 2-(Norborn-5-enyl)ethyldimethylchlorosilane

5-Vinyl-2-norbornene (4.20 M, 504 g) was stirred under nitrogen at70°-75° C. in a round-bottomed flask equipped with a thermometer, anefficient condenser and a constant pressure addition funnel.Dimethylchlorosilane (2.8 M, 264 g) was added dropwise and a Karstedtcatalyst (0.77 g) was added to the reaction mixture. When the additionwas complete, the reaction mixture was aged at 70° C. for two hours.Analysis of the reaction indicated no silane present (FT-IR). Themixture was then stripped on a rotary evaporator and distilled in vacuo(bp 60°-64° C., 0.05 mm Hg). Yield 493 g (82% theory, 94% purity byGLPC).

EXAMPLE 3 Ethyl Norbornene capped polyorganosiloxane

Silanol fluid, Rhone Poulenc 48V3500, (0.092 eq OH) was stirred undernitrogen and warmed to 70° C. with triethylamine-dimethylaminopyridine(1.05 eq, 1% DMAP by weight). The product of example 2 (0.0956 eq, 20.5g) was added. The reaction mixture was stirred at 75° C. for three hoursthen methanol (50 g) was added slowly. After one hour, volatiles wereremoved by concentration on a rotary evaporator, the reaction mixturewas diluted with hexane and filtered through diatomaceous earth. Removalof solvent on a rotary evaporator at 75° C. (0.1 mm Hg) gave theproduct: Viscosity 4680 cps, GPC MW_(wt) 30787, MW_(number) 23943.

EXAMPLE 4 Norbornene Cyclic Tetramer 32 Water (200ml, 11.1 moles) wasadded dropwise to 200.0g, 0.85 mole,2-(norborn-5-enyl)ethyldichloromethylsilane in 200 ml THF in a 1 liter3-necked flask with thermometer and mechanical stirrer, allowing gentleTHF reflux at 65° C. during the addition. The contents were stirred at70° C. for one hour after the addition, then allowed to phase. The upperproduct layer was separated, then added to 100 g water and 50 gpotassium carbonate with stirring for one hour at 70° C. After allowingto phase, the upper, product layer was removed, concentrated on a rotaryevaporator, rediluted with an equal volume of hexane and filteredthrough a Celite® pad. The product was isolated by rotary evaporation,giving 160.4 g of a hazy, light yellow oil (89% Theory, MW_(wt) =735,MW_(number) =607 by GPC, absence of SiOH confirmed by IR). BrookfieldViscosity: 14,300 cps (RV4 spindle, 5 rpm). EXAMPLE 5

Using a premixed base of the product of example 3 with 35% HDK 2000™silica and 1.5% diethoxyacetophenone (DEAP), the inventors combinedvarious thiol equivalent percent (eq %) increments of 3K5M (a polythiolof formula B where x is about 5 and y is about 30) and1,3-bis(3-mercaptopropyl)-1,1,3,3-tetramethyldisiloxane (a polythiol offormula VII referred to herein as "C") adding up to the totalunsaturation in the premix.

The formulations were cast between polyethylene-backed 6" square steelplates with a 70 mil by 6" square outer dimension by 5 1/2" square innerdimension steel spacer. The polyethylene-backed specimens were clampedbetween two 6" square glass plates and placed in a Technocure™ highpressure mercury vapor lamp UV curing chamber and irradiated 60 secondsper specimen side at an intensity of 70 milliwatts per squarecentimeter. After cooling, the cured specimen was peeled free of theplates and cut into tensile and tear test specimens per ASTM MethodD412. The following bulk properties were obtained on the test specimens:

                  TABLE I                                                         ______________________________________                                        EFFECT OF 1,3-BIS(3-MERCAPTOPROPYL)-1,1,3,3-                                  TETRAMETHYLDISILOXANE                                                                  Amounts of C                                                                  20 eo %                                                                              40 eq %   60 eq %  80 eq %                                    ______________________________________                                        Premix     150 g    150 g     150 g  150 g                                    3K5M       5.8 g    4.3 g     2.9 g   1.45 g                                  (C)        0.3 g    0.6 g     0.9 g   1.2 g                                   Tensile (psi)                                                                            1215     1254      931    269                                      Elongation (%)                                                                            454      589      701    630                                      Tear (pli)  218      231      186     37                                      Shore A     40       37        27     13                                      % Extractables                                                                           4.5      6.3       8.1    16.5                                     ______________________________________                                    

These data indicate that nearly equiequivalent amounts of 3K5M and theoligomeric siloxane dithiol "C" offer the best elongation improvementversus extent of cure. The 80 eq % point, illustrates the effects ofvery low crosslink density in this system, evidenced by the low Shore A.Cured properties without any (C) are very similar to the 20 eq % point.

It is readily apparent that (C) adds additional flexibility to the aboveformulations.

EXAMPLE 6

The propargyl ester of 2-norborn-5-ene carboxylic acid was prepared byDiels-Alder reaction of propargyl acrylate and cyclopentadiene atambient temperature. Dimethylchlorosilane (56.4g) was added portionwiseto 100 grams of this norbornene carboxylate ester and 0.16 g Karstedtcatalyst under nitrogen at 70° C. After exotherming to 80° C., themixture was stirred for 3 hours at 70° C., stripped under light vacuumand then distilled. The product, ##STR11##

EXAMPLE 7

A mixture of allyl alcohol (116 g, 2.0 mol), triethylamine (202 g, 2.0mol) and 4-dimethylaminopyridine (2.44 g, 0.02 mol) in benzene (500 ml)was stirred at 51° C. in a 2 liter three-necked flask equipped with amechanical stirrer, a thermometer, and a pressure equalizing additionfunnel under a nitrogen atmosphere. A solution ofnorborn-5-ene-2-carboxylic acid chloride (197 g, 1.26 mol) in benzene(100 ml) was added dropwise so that the temperature of the reaction didnot exceed 20° C. during the addition. When the addition was complete,the viscous slurry was warmed to 55° C. and aged at this temperature forone hour. The reaction mixture was then cooled to room temperature andfiltered. The filter cake was washed successively with pentane (2 times200 ml) and ether (2 times 200 ml). The filtrate was then concentratedon a rotary evaporator and distilled in vacuo (bp 85°-93° C., at 0.2-0.1mm Hg). The yield of distilled product was 201 g and consisted of amixture of endo and exo isomers of allyl norborn-5-ene™2-carboxylate.The purity of the product was judged to be >98% by gas chromatographicanalysis.

Chlorodimethylsilane (26.6 g, 0.281 mol) was added slowly at 80° C. to50.0 g (0.281 mol) allyl norborn- 5-ene-2-carboxylate and 0.16 gKarstedt catalyst (50 ppm Pt) in 100 ml toluene under a nitrogenblanket. No exotherm was observed, even with complete addition ofchlorodimethylsilane. After heating and stirring 18 hours at 80° C., IRconfirmed the disappearance of SiH. After briefly stripping on a rotaryevaporator, a small quantity of precipitate formed, which was removed bydilution with an equal volume of heptane, and filtering. Afterreconcentrating, the crude product was vacuum distilled, removing aforecut at 50°-90° C. at 0.06 mm, followed by the product at 90°-92° C.at 0.06 mm. Yield: 30.6 g (40%) of a slightly cloudy liquid. MNRanalysis confirmed the product as: ##STR12## with no evidence of silaneaddition to the internal ring norbornene double bond.

EXAMPLE 8

One hundred grams of an ethylnorbornene capped polyorganosiloxane asprepared in Example 3 was mixed in a 250 ml beaker with 3.91 gm of a SiHfunctional polydimethylsiloxane having an average of 7 SiH groups permolecule and an SiH equivalent wt. of 346, and a catalytic amount ofKarstedt catalyst (0.1 gm). The liquid mixture gelled within 3 hours andproduced a rubbery elastomeric solid easily removable from the beakerafter 24 hours at ambient temperature.

What is claimed is:
 1. A composition curable to a crosslinkedpolyorganosiloxane the composition comprising:a) A silicone prepolymerhaving a plurality of norbornene functional groups of the formula:##STR13## bound to silicon atoms thereof, where R is H or methyl, R¹ isa direct bond or --C(═O)O-- and R⁴ is ethylene, ethylene, propylene orpropenylene, b) a coreactive alkylthiol functional silicone prepolymerhaving a plurality of alkylthiol functional groups, said siliconeprepolymers (a) and (b) being present in an equivalent ratio of between2:1 and 1:2 norbornene to alkylthiol groups, and c) an effective amountfor cure initiation of a thiol-ene cure catalyst.
 2. A composition as inclaim 1 wherein the said catalyst is a free radical initiator.
 3. Aformulation as in claim 2 wherein the initiator is a free radicalphotoinitiator.
 4. A formulation as in claim 2 wherein the norbornenefunctional silicone prepolymer has the formula: ##STR14## where q isfrom 100-1500.
 5. A formulation as in claim 4 wherein the norbornenefunctional silicone prepolymer and the alkylthiol functional siliconeprepolymer are present in about equal amounts on an equivalents basis.6. A formulation as in claim 1 wherein the alkylthiol functionalsilicone prepolymer component comprises between about 20% and 80%, onthe basis of total equivalents of alkylthiol, of a compound of theformula: ##STR15## where m is between 1 and 3 and n is 3-15.
 7. Aformulation as in claim 1 wherein the coreactive silicone prepolymercomponent is a mixture of alkylthiol functional silicones which consistsessentially of a polythiol compound of the formula: ##STR16## where R⁷is lower alkylene, x is 3-10 and y is 25-50 and an oligomeric dithiolchain extender of the formula: ##STR17## where m is between 1 and 3 andn is 3-15, said polythiol being present in an amount of at least 20% ofthe total number of equivalents of the alkylthiol component.
 8. Aformulation as in claim 7 wherein R⁷ is ethylene, m is 1 and n is 3-4.9. A composition as in claim 1 wherein the norbornene functionalsilicone prepolymer is substantially free of groups of the formula:##STR18## bound to silicon atoms thereof where R and R¹ are aspreviously described and R² is vinyl, ethynyl, allyl or propargyl.
 10. Acomposition as in claim 1 wherein R¹ is a direct bond and R⁴ isethylene.
 11. A composition as in claim 1 wherein R¹ is --C(═O)O-- andR⁴ is a propylene or propenylene group.
 12. A composition as in claim 1wherein the total reactive functionality of the composition is greaterthan
 4. 13. A formulation as in claim 1 wherein at least 20% of thealkylthiol functional silicone prepolymer, based on the total number ofthiol equivalents, comprises a polythiol compound of the formula:##STR19## where R⁷ is lower alkylene, x is 3-10 and y is 25-50.
 14. Apolymer produced by curing a composition as in claim
 2. 15. A polymerproduced by curing a composition as in claim 7.